The quantitative determination of analytes in body fluids is of great importance in the diagnoses and maintenance of certain physical conditions. For example, lactate, cholesterol and bilirubin should be monitored in certain individuals. In particular, it is important that individuals with diabetes frequently check the glucose level in their body fluids to regulate the glucose intake in their diets. The results of such tests can be used to determine what, if any, insulin or other medication needs to be administered. In one type of blood-glucose testing system, test sensors are used to test a sample of blood.
A test sensor contains biosensing or reagent material that reacts with, for example, blood glucose. One type of electrochemical test sensor is a multilayer test sensor that includes a base or substrate and a lid. Another type of electrochemical test sensor includes a base, a spacer and a lid. Existing electrochemical test sensors include at least two electrodes in the form of an electrode pattern. A potential is applied across these electrodes and a current is measured at the working electrode. The current measurement is directly proportional to the size of the working electrode.
Electrochemical test sensors are based on enzyme-catalyzed chemical reactions involving the analyte of interest. In the case of glucose monitoring, the relevant chemical reaction is the oxidation of glucose to gluconolactone or its corresponding acid. This oxidation is catalyzed by a variety of enzymes, some of which may use coenzymes such as nicotinamide adenine dinucleotide (phosphate) (NAD(P)), while others may use coenzymes such as flavin adenine dinucleotide (FAD) or pyrroloquinolinequinone (PQQ).
In test-sensor applications, the redox equivalents generated in the course of the oxidation of glucose are transported to the surface of an electrode, whereby an electrical signal is generated. The magnitude of the electrical signal is then correlated with glucose concentration. The transfer of redox equivalents from the site of chemical reaction in the enzyme to the surface of the electrode is accomplished using electron transfer mediators.
Many mediators such as, for example, ferricyanide have a high background current such that the signal-to-noise ratio when formulated in a glucose test sensor is low. Typically, a low signal-to-noise ratio results in a higher assay imprecision, particularly at lower glucose levels and high hematocrit sample levels. With quicker sample tests (e.g., test times less than 10 seconds), it may be difficult to burn off the high background current in the time allocated to perform the test. Because of the quicker sample test times, this necessitates that the active ingredients interact rapidly when sample is applied to give a rapid response.
Therefore, it would be desirable to form a mediator that has a low background current, while still having other desirable attributes of a mediator including stability.